Come back!

Did that title scare you?  Precession (no, I’m not misspelling it) is a concept which looks confusing at first glance, but really isn’t.  It’s a bit bulky, but hang in there.

First, think of a spinning top that has a bit of a wobble on top… like this:

Precession of a gyroscope (spinning top)

Now, substitute in everyone’s favorite astronomical spinning top that has a bit of a wobble on top:

Precession of another gyroscope, image found at perceptions.couk.com

As the Earth wobbles around, making that cone shape, it changes its orientation to the stars; you can see how Magnetic North wobbles between Vega and Polaris during the cycle.  That’s precession; it’s the changing orientation of the Earth to the stars as it makes that wobble.  The Earth goes through a complete cycle about every 26,000 years (or 25,771.5, if you’re picky).  This is why there’s a difference in the tropical year and the sidereal year.  The tropical year is about 20 minutes shorter than the sidereal year.  Confused?  Don’t be.  A tropical year is one year measured from solstice to solstice (or equinox to equinox), and a sidereal year is one year measured from the Sun’s apparent position relative to a specific star, and back again.

Very nice image from Calgary.rasc.ca - posted to public domain - copyright, all rights reserved

This westward movement of the equinoxes along the ecliptic was historically called “Precession of the Equinoxes”.  Its discovery is credited to Hipparchos around 147-127 BCE.  He was a Greek astronomer and mathematician.   You can blame him for trigonometry.  Anyway, Hipparchos took very specific measurements of the lengths of the tropical year and the sidereal year, and because of the discrepancy (that 20 minute difference) he concluded that the equinoxes were moving (“precessing”) through the Zodiac.

Ptolemy, another astronomer and mathematician, compared his own work to that of Hipparchos (a 265-year difference), and confirmed precession of the equinoxes.  In his time, Ptolemy’s work was viewed with derision by many because he opined that a person’s environment and upbringing had more influence on their personality than the position of the stars and planets at the moment of their birth.  I think the word I’m searching for here is “heresy”.

Very nice image showing the cycle of the North Pole - image from my.execpc.com - all rights reserved

Some researchers believe the Babylonian astronomers were kicking around the idea of precession as early as 330 BCE.  The Babylonians were studying the position of the stars and constellations for their astronomical catalogs by the First Dynasty, ca. 1830 BCE.  I’m sure they noticed something was going on.

It’s not known for sure when the Ancient Egyptians became aware of precession, but it was probably fairly early on.  Likewise the Indian and Chinese astrologers.  The belief that the Mayan Long Count is calibrated against the precession is not backed up by most professional Mayan scholars.

In astronomy the word “precession” itself refers to several types of continuous, gravity-induced changes in a celestial body. The woods are full of precessions.  There is nodal precession, lunar precession, de Sitter precession, apsidal precession… well, you get the idea.  It’s bulky, but you’ve had the concept down cold since you were a child playing with a spinning top.

Isn’t it amazing how much you already know?